Filed under: News
Feb 27 2014, 2:51am CST | by Forbes
For the first time, a team of American astronomers has used near-infrared spectroscopy to directly detect water vapor in the atmosphere of a gas giant planet in close orbit around the bright star tau Bootis — one that according to Homer may have even led Odysseus home from Troy.
But not even Odysseus could have imagined that 21st century spectroscopy would be teasing data from the hellishly hot atmosphere of a 3.8 Jupiter-mass planet around a star only 50 light years from earth.
While water has been detected in the atmospheres of several extrasolar planets using other techniques, this detection via thermal emission, as reported in The Astrophysical Journal Letters enables astronomers to directly characterize the atmospheres of such “hot Jupiters,” or gas giant planets that orbit their host stars on astoundingly close orbits.
“This discovery of water on tau Bootis b is absolutely wonderful,” said longtime extrasolar planet hunter Geoff Marcy, an astronomer at the University of California at Berkeley, who along with astronomer Paul Butler, first discovered the planet in 1996.
And it’s all the more “incredible,” says Marcy, considering that only 18 years ago, he and colleagues still thought it “a miracle” to be able to indirectly detect any such extrasolar Jupiter-mass planets, much less study their atmospheres.
Penn State University astronomer Chad Bender, one of the paper’s co-authors, says this is the first time anyone has detected water in a non-transiting planet. That is, a planet having its atmosphere probed by the background glow from its parent star.
That’s important, says Bender, because the population of non-transiting extrasolar planets is much larger than those that from our line of sight appear to transit across the face of their parent stars.
In 2011, Bender and colleagues used the Keck II 10.2-meter telescope atop Mauna Kea in Hawaii to take spectra of the planet on five separate nights.
“We took blended spectra from both the star and the planet,” said Bender. “The trick was separating the chemical signatures in that spectra — the ones that belonged to the star and the ones that belonged to the planet.”
Because the planet is about 10,000 times fainter than the star, says Bender, the star’s light completely overwhelms the planet’s spectra. But he says the near infrared spectrum he and colleagues took was done at high resolution which enabled the team to detect several molecular lines of spectra; which, in this case, included water.
Bender stressed that the spectra they received was not from the planet’s albedo (or reflected light from its parent star), but rather light being generated from the planet’s atmospheric heat, since it orbits its parent star tau Bootis once every 3.3 days.
The hope is that astronomers will be able to use the same technique to take spectra of much cooler planets; ones that aren’t locked in such tight orbits around their parent stars.
But is it possible to really take spectra of much cooler planets; ones that aren’t locked in tight orbits around their parent stars?
“Observing earthlike planets in the habitable zone is going to be difficult because they are going to have atmospheres at earthlike temperatures,” said Bender. “That means the contrast between the planet and the star is really very extreme — something like ten orders of magnitude in difference.”
That, says Bender, will at least require the next generation of large telescopes, such as NASA’s scheduled James Webb Space Telescope (JWST) or Caltech’s Thirty Meter Telescope (TMT), a planned ground-based behemoth.
Thus, although water appears to be ubiquitous, Benders admits the technology still has a long way to go before it will be sensitive to earths in a habitable zone, where liquid water would be present.
But as Marcy notes: “This discovery reminds us that hydrogen and oxygen are so abundant in the universe that H2O must be ubiquitous. Considering that water is the ‘sine qua non’ of life, this bodes well for the possibility of other critters that dream of galactic travel and exploration.”
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